MOSAIC in a nutshell

MOSAIC is a multi-object and multi-integral field spectrograph that will use the widest possible field of view provided by the ELT. The MOSAIC top-level requirements have been based on a comprehensive White Paper summarizing the very numerous scientific cases for a multi-object spectrograph on the ELT. MOSAIC will have three operating modes: a high multiplex mode (HMM) covering the visible and near-infrared domain; a high definition mode (HDM) that will provide spatially resolved observations in the near-infrared; and a multi light bucket integral field mode for the Inter-Galactic Medium mode (IGM).

High multiplex mode (HMM)

This mode allows us to observe simultaneously 200 astronomical objects in the visible domain, or 100 in near infrared, in a single exposure.

Science drivers: Resolved stellar population of local group galaxies and astro-archeology (SC #4 and #5), the origin of dwarf galaxies (SC #3)

Why multiplex ? Even with the unbeatable collecting power of the ELT, the observed astronomical sources are so faint, that it will require several hours to obtain spectra. The high multiplex mode enables us to observe hundreds of targets simultaneoulsy in a single exposure of a few hours (1h at ELT). Without multiplex capability, i.e. possibility to observe several objects at the same time, the same observations would require several hundreds of hours (100h).

High definition mode (HDM)

Integral field units (IFUs) are used in this mode, combined with the multi-object adaptive optics system (MOAO). It allows to obtain spatially resolved observations at high definition for 10 astronomical objects in the ELT field of view.

This technology combines spectrographic and imaging capabilities. The integral field unit slices the astronomical object into several regions, each of them being spectrographied. This leads to spatially resolved observations of the object, by providing a spectrum for each region;

Instrument build by the consortium: GIRAFFE@VLT, KMOS@VLT, MUSE@VLT

MOSAIC Conceptual design

The design is based on two principles: first, MOSAIC will have a shared focal-plate with multi-function tiles which can serve as pick-offs for any of the modes and AO functions, and second, it will include shared-slit spectrographs for which the spectrograph optics and detectors can be used either for the high-multiplex mode or for the IFU mode.

Shared focal-plate

MOSAIC focal plane is filled by hexagonal tiles. On each tile there will be a locally controled positioner that allocates for the HMM apertures and for the HDM pick-off mirror. Figure below shows a conceptual design for the MOSAIC focal plane and the implementation of the 4 observational modes.

The two high multiplex modes (HMM) will operate in seeing limited or ground layer adaptive optics (GLAO) conditions with the following specifications:

HMM-VIS: 200 sub-fields of 0.80” in diameter to be allocated within a 32 arcmin^2 field. Each sub-field consists in bundles of several microlens + fibres.

The two integrated field spectroscopy modes HMM and IGM will operate with the following specifications:

HDM: High definition mode, operating with multi object adaptive optics (MOAO) in the near-IR. A pick- off mirror in the focal plane directs light via an MOAO adaptive system (receiver) and fibre bundle to the spectrograph

MOSAIC is the proposed multiple-object spectrograph for the ELT that will utilise the widest possible field of view provided by the telescope. In terms of adaptive optics, there are two distinct operating modes required to meet the top-level science requirements. The MOSAIC High Multiplex Mode (HMM) requires either seeing-limited or GLAO correction within a 0.6 (NIR) and 0.9 (VIS) arcsecond sub-fields over the widest possible field for a few hundred objects. To achieve seeing limited operation whilst maintaining the maximum unvignetted field of view for scientific observation will require recreating some of the functionality present in the Pre-Focal Station relating to control of the ELT active optics. MOSAIC High Definition Mode Control (HDM) requires a 25% Ensquared Energy (EE) within 150mas in the H-band element for approximately 10 targets distributed across the full ELT field, implying the use of Multiple Object AO (MOAO). Initial studies have shown that to meet the EE requirements whilst maintaining high-sky coverage will require the combination of wavefront signals from both high-order NGS and LGS to provide a tomographic estimate for the correction to be applied to the open-loop MOAO DMs. In this paper we present the current MOSAIC AO design and provide the first performance estimates for the baseline instrument design. We then report on the various trade-offs that will be investigated throughout the course of the Phase A study, such as the requirement to mix NGS and LGS signals tomographically. Finally, we discuss how these will impact the AO architecture, the MOSAIC design and ultimately the scientific performance of this wide-field workhorse instrument at the ELT.

Positioner System Architecture

Baseline for the full architecture is a stepped tiled focal plane. This is the only configuration which can compensate adequately for the non-telecentricity of the ELT and provide support for all required modes. With 7.4’, and a tile size of 0.5’ (~100mm), we can achieve the 200 multiplex including LGS vignetting (TBD)

REM : this is just a concept representative of the type of tile we envision for MOSAIC.

Both HMM and HDM modes use microlenses and fibres to transmit light to the spectrographs, the principle being to make a pupil image on the fibre cores. Microlenses and fibres are directly on the tile in the focal plate for HMM, while they are offset from the focal plate, after an optical relay including a deformable mirror for HDM. In the case of HDM, the microlens surface is conjugated to the image of the sky, and acts as a high-definition sampler.